Comparison of linear, hyperbolic and double-hyperbolic models to assess the force-velocity relationship in multi-joint exercises

The Rate of Torque Development as a Determinant of the Torque-Velocity Relationship

We investigate the contribution of isometric rate of torque development (RTD) and maximal voluntary torque (MVT) to the dynamic force production capacities of knee extensors obtained from the torque-velocity (TV) relationship, that is, the theoretical maximal velocity (V0), torque (T0), and maximal power (Pmax). Single-leg knee extensors were tested in 64 young adults (31 females). RTD and root mean square (RMS) of electromyographic signals from the knee extensors were recorded during isometric and incremental load dynamic (nonisokinetic) contractions. In the dynamic test, torque and velocity were continuously measured and averaged over 80°-140° knee angles to determine individual TV relationships. TV relationships were well fitted by hyperbolic regression (r2 from 0.983 to 0.993). Stepwise linear regressions showed that the main determinant of V0 was normalized RTD50 (R2 = 0.145, p = 0.004); the main determinant of T0 was MVT (R2 = 0.760, p < 0.001); and the main determinant of Pmax was RTD150 (R2 = 0.612, p < 0.001). V0 (when obtained from averaged values over knee extension) is partially explained by rapid torque capacity (i.e., "explosive strength"). Therefore, the capacity to produce torque at high velocity partly depends on the capacity to rise quickly the torque in the early phase of the contraction, suggesting that some underlying determinants of RFD would also affect V0.

Exerting force at the maximal speed drives the increase in power output in elite athletes after 4 weeks of resistance training

PURPOSE

In the present study, we examined how a 4-week intervention of maximal intended velocity (MIVRT) and controlled velocity resistance training (CRT)-induced task-specific responses in expert individuals.

METHODS

Twenty elite athletes were randomly assigned to either a MIVRT (n = 10) or CRT (n = 10) group, both following the same volume-load training based on the back-squat three times a week but with different intentions in moving load (force-exertion speed). We assessed one-repetition maximum (1RM), mean propulsive velocity (MPV), and mean propulsive power (MPP) using a progressive-loading test before and after the intervention. A linear position transducer was used to monitor propulsive velocity in training and testing sessions.

RESULTS

Both groups significantly increased their 1RM (CRT: + 12.3%, p < 0.001, d = 0.39; MIVRT: + 12.5%, p < 0.001, d = 0.45). Only the MIVRT group showed a significant improvement in MPV (p < 0.01) across different stepping loads, while both groups improved in MPP (MIVRT: + 22.4%, p < 0.001, d = 0.54; CRT: + 8.1%, p = 0.04, d = 0.17).

CONCLUSIONS

MIVRT induced significant adaptations in MPV and MPP at various loads (%1RM), underlining its specificity in targeting these parameters. Despite similar enhancements in 1RM, the distinct training protocols suggest that strength gains may stem from either maximal intent in moving loads or longer times under tension. This study highlights the role of execution speed in optimizing power outcomes, emphasizing task specificity as paramount to elicit physiological adaptations in chronically strength-trained individuals.

Mechanisms of myosin II force generation: insights from novel experimental techniques and approaches

Myosin II is a molecular motor that converts chemical energy derived from ATP hydrolysis into mechanical work. Myosin II isoforms are responsible for muscle contraction and a range of cell functions relying on the development of force and motion. When the motor attaches to actin, ATP is hydrolyzed and inorganic phosphate (Pi) and ADP are released from its active site. These reactions are coordinated with changes in the structure of myosin, promoting the so-called "power stroke" that causes the sliding of actin filaments. The general features of the myosin-actin interactions are well accepted, but there are critical issues that remain poorly understood, mostly due to technological limitations. In recent years, there has been a significant advance in structural, biochemical, and mechanical methods that have advanced the field considerably. New modeling approaches have also allowed researchers to understand actomyosin interactions at different levels of analysis. This paper reviews recent studies looking into the interaction between myosin II and actin filaments, which leads to power stroke and force generation. It reviews studies conducted with single myosin molecules, myosins working in filaments, muscle sarcomeres, myofibrils, and fibers. It also reviews the mathematical models that have been used to understand the mechanics of myosin II in approaches focusing on single molecules to ensembles. Finally, it includes brief sections on translational aspects, how changes in the myosin motor by mutations and/or posttranslational modifications may cause detrimental effects in diseases and aging, among other conditions, and how myosin II has become an emerging drug target.

The linear regression model provides the force-velocity relationship parameters with the highest reliability

An a-posteriori multicentre reliability study was conducted to compare the between-session reliability of the force-velocity relationship parameters (force-intercept [F0], velocity-intercept [v0], and maximum power [Pmax]) between different regression models during the bench press (BP) and bench press throw (BPT) exercises. Data from four and three studies were considered for the BP (n = 102) and BPT (n = 81) exercises, respectively. The force-velocity relationships were determined using five regression models: linear multiple-point, linear two-point, quadratic polynomial, hyperbolic, and exponential. All regression models provided F0 and Pmax with acceptable reliability (cut-off CV ≤ 9.45%; cut-off ICC ≥ 0.79) with the exceptions of F0 for the quadratic polynomial and hyperbolic models (BPT) and Pmax for the exponential model (BP and BPT). Only the linear multiple- and linear two-point models provided v0 with acceptable absolute reliability (cut-off CV ≤ 7.72%). Regardless of the exercise, the reliability of the three parameters was generally higher for the linear multiple- and two-point models compared to the other models (CVratio ≥ 1.22), and no significant differences were observed between multiple- and linear two-point models (CVratio ≤ 1.11). Linear regression models are recommended to maximise the reliability of the force-velocity relationship parameters during the BP and BPT exercises.

Age-Related Differences and Reliability of a Field-Based Fitness Test Battery in Young Trained Footballers: The Role of Biological Age

BACKGROUND

the purpose of this study was to analyze the reliability of a field-based fitness test battery in young trained football players, according to biological age.

METHODS

197 young trained football players (12-19 years old) participated in the study. We measured anthropometric measurements (i.e., height, sitting height, length leg, and body mass), a bilateral vertical jumping test (CMJ), a progressive loading test of squats and hip thrust, acceleration and speed tests (10 m and 30 m sprint tests), a change-of-direction ability test (V-cut test), and a cardiorespiratory fitness test (30-15 intermittent fitness test). Statistical data are shown as the mean ± standard deviation by PHV group in tests and retests. Test reliability was assessed through the intraclass correlation coefficient (ICC), with an ICC above 0.9 being considered high. To evaluate accuracy and repeatability, standard error of measurement, coefficient of variation, and minimum detectable change at 90% were determined and Bland-Altman diagrams were used, establishing a statistical significance of p < 0.05.

RESULTS

All of the tests showed non-significant differences between the test and retest in the pooled sample (p > 0.05). Furthermore, all of them presented a trivial effect size (<0.2) and high intraclass correlation coefficients (>0.9), which indicates the high reproducibility of the tests, despite some of them presenting a significant difference between trials (i.e., the CMJ, 10 m sprint, V-cut, and squat tests). Low measurement errors were found for all tests (coefficient of variation [CV] = 4.39-9.39), except for the CMJ and the progressive loading test for squat and hip thrust exercises (CV = 12.2-21.11). Similar results were found irrespective of biological age group.

CONCLUSIONS

All tests were reliable for the pooled sample as well as for the biological age groups.

Vastus Lateralis Muscle Size Is Differently Associated With the Different Regions of the Squat Force-Velocity and Load-Velocity Relationships, Rate of Force Development, and Physical Performance Young Men

ABSTRACT

Cornejo-Daza, PJ, Sánchez-Valdepeñas, J, Rodiles-Guerrero, L, Páez-Maldonado, JA, Ara, I, León-Prados, JA, Alegre, LM, Pareja-Blanco, F, and Alcazar, J. Vastus lateralis muscle size is differently associated with the different regions of the squat force-velocity and load-velocity relationships, rate of force development, and physical performance young men. J Strength Cond Res 38(3): 450-458, 2024-The influence that regional muscle size and muscle volume may have on different portions of the force-velocity (F-V) and load-velocity (L-V) relationships, explosive force, and muscle function of the lower limbs is poorly understood. This study assessed the association of muscle size with the F-V and L-V relationships, rate of force development (RFD) and maximal isometric force in the squat exercise, and vertical jump performance via countermovement jump (CMJ) height. Forty-nine resistance-trained young men (22.7 ± 3.3 years old) participated in the study. Anatomical cross-sectional area (ACSA) of the vastus lateralis (VLA) muscle was measured using the extended field of view mode in an ultrasound device at 3 different femur lengths (40% [distal], 57.5% [medial], and 75% [proximal]), and muscle volume was estimated considering the VLA muscle insertion points previously published and validated in this study. There were significant associations between all muscle size measures (except distal ACSA) and (a) forces and loads yielded at velocities ranging from 0 to 1.5 m·s -1 ( r = 0.36-0.74, p < 0.05), (b) velocities exerted at forces and loads ranging between 750-2,000 N and 75-200 kg, respectively ( r = 0.31-0.69, p < 0.05), and (c) RFD at 200 and 400 milliseconds ( r = 0.35-0.64, p < 0.05). Proximal and distal ACSA and muscle volume were significantly associated with CMJ height ( r = 0.32-0.51, p < 0.05). Vastus lateralis muscle size exhibited a greater influence on performance at higher forces or loads and lower velocities and late phases of explosive muscle actions. Additionally, proximal ACSA and muscle volume showed the highest correlation with the muscle function measures.

Examining Unilateral and Bilateral Exercises through the Load-velocity Relationship

This study aimed to examine the load-velocity (L-V) relationship in the bench-press (BP) and leg-press (LP) exercises performed unilaterally, and compared this unilateral L-V relationship with the bilateral variants. Nineteen men (age=23.5±2.1 years) completed two incremental tests in BP and LP, performed bilaterally and unilaterally, across two sessions with a 48-hour rest period. We found a close relationship between medium propulsive velocity (MPV) and %1RM in unilateral BP (R2  =0.97, SEE=0.06 m·s-1) and LP (R2=0.96, SEE=0.06 m·s-1). No significant differences were observed between the preferred and non-preferred sides in the L-V relationship for either exercise. Additionally, higher velocities were achieved in unilateral exercises compared to bilateral exercises, particularly with light and moderate loads (30-70%1RM) in BP (p<0.05) and with light loads (30-45%1RM) in LP (p<0.05). Close L-V relationships were observed in unilateral exercises, without differences in the L-V relationships between preferred and non-preferred sides despite the interlimb asymmetries in the absolute strength values. Interestingly, lower velocities were observed at light loads (~30-45% 1RM) for bilateral compared to unilateral exercises, which could be explained by different strength deficits for these exercises.

The 2-Point Method: Theoretical Basis, Methodological Considerations, Experimental Support, and Its Application Under Field Conditions

The "2-point method," originally referred to as the "2-load method," was proposed in 2016 by Prof Slobodan Jaric to characterize the maximal mechanical capacities of the muscles to produce force, velocity, and power. Two years later, in 2018, Prof Jaric and I summarized in a review article the scientific evidence showing that the 2-point method, compared with the multiple-point method, is capable of providing the outcomes of the force-velocity (F-V) and load-velocity (L-V) relationships with similar reliability and high concurrent validity. However, a major gap of our review was that, until 2018, the feasibility of the 2-point method had only been explored through testing procedures based on multiple (more than 2) loads. This is problematic because (1) it has misled users into thinking that implementing the 2-point method inevitably requires testing more than 2 conditions and (2) obtaining the data from the same test could have artificially inflated the concurrent validity of the 2-point method. To overcome these limitations, subsequent studies have implemented in separate sessions the 2-point method under field conditions (only 2 different loads applied in the testing protocol) and the standard multiple-point method. These studies consistently demonstrate that while the outcomes of the 2-point method exhibit comparable reliability, they tend to have slightly higher magnitudes compared with the standard multiple-point method. This review article emphasizes the practical aspects that should be considered when applying the 2-point method under field conditions to obtain the main outcomes of the F-V and L-V relationships.

Ten-year longitudinal changes in muscle power, force, and velocity in young, middle-aged, and older adults

BACKGROUND

Maximum muscle power (Pmax ) is a biomarker of physical performance in all ages. No longitudinal studies have assessed the effects of aging on Pmax obtained from the torque-velocity (T-V) relationship, which should be considered the 'gold standard'. This study evaluated the longitudinal changes in the T-V relationship and Pmax of the knee-extensor muscles in young, middle-aged, and older adults after 10 years of follow-up.

METHODS

Four hundred eighty-nine subjects (311 men and 178 women; aged 19-68 years) were tested at baseline and after a 10-year follow-up. Anthropometric data, daily protein intake, physical activity level (PAL), and knee-extension muscle function (isometric, isokinetic, and isotonic) were evaluated. A novel hybrid equation combining a linear and a hyperbolic (Hill-type) region was used to obtain the T-V relationship and Pmax of the participants, who were grouped by sex and age (young: 20-40 years; middle-aged: 40-60 years; and old: ≥60 years). Linear mixed-effect models were used to assess effects of time, sex, and age on T-V parameters, Pmax , and body mass index (BMI). Additional analyses were performed to adjust for changes in daily protein intake and PAL.

RESULTS

Pmax decreased in young men (-0.6% per year; P < 0.001), middle-aged men and women (-1.1% to -1.4% per year; P < 0.001), and older men and women (-2.2% to -2.4% per year; P ≤ 0.053). These changes were mainly related to decrements in torque at Pmax at early age and to decrements in both torque and velocity at Pmax at older age. BMI increased among young and middle-aged adults (0.2% to 0.5% per year; P < 0.001), which led to greater declines in relative Pmax in those groups. S/T0 , that is, the linear slope of the T-V relationship relative to maximal torque, exhibited a significant decline over time (-0.10%T0 ·rad·s-1 per year; P < 0.001), which was significant among middle-aged men and old men and women (all P < 0.05). Annual changes in PAL index were significantly associated to annual changes in Pmax (P = 0.017), so the overall decline in Pmax was slightly attenuated in the adjusted model (-5.26 vs. -5.05 W per year; both P < 0.001).

CONCLUSIONS

Pmax decreased in young, middle-aged, and older adults after a 10-year follow-up. The early declines in Pmax seemed to coincide with declines in force, whereas the progressive decline at later age was associated with declines in both force and velocity. A progressively blunted ability to produce force, especially at moderate to high movement velocities, should be considered a specific hallmark of aging.

The Use of Incremental, Decremental or a Random Order of Loads does not Affect Peak velocity Values during Bench Press Throw Load-Velocity Relationships

The aim of this study was to assess the influence of the load order used (i. e. incremental, decremental or random loads order) during the bench press throw load-velocity profile on peak velocity achieved against four different loads (20-40-60-80% of one repetition maximum [1RM]). Both intraclass correlation coefficient (ICC) and coefficient of variation (CV) were calculated to assess the reliability of the measures. A repeated measures ANOVA was used to assess differences between protocols. A linear regression analysis was performed to assess the load-velocity relationships among the different protocols. Peak velocity showed good to high ICC values independently of the load used (ICC=0.83-0.92). CV scores showed good reliability (ranging between 2.2 and 6.2%). No significant differences in peak velocity attained at each load were found between the three testing protocols (p>0.05). In addition, peak velocity at each load was very large to almost perfect, correlated between protocols (r=0.790-0.920). The linear regression model showed a significant relationship between testing protocols (p<0.001; R²=0.94). In conclusion, due to some ICC scores below 0.9 and R² below 0.95, the indistinct use of different load-order protocols to assess load-velocity relationships in the bench press throw exercise is not recommended.

Toward a New Paradigm in Resistance Training by Means of Velocity Monitoring: A Critical and Challenging Narrative

For more than a century, many concepts and several theories and principles pertaining to the goals, organization, methodology and evaluation of the effects of resistance training (RT) have been developed and discussed between coaches and scientists. This cumulative body of knowledge and practices has contributed substantially to the evolution of RT methodology. However, a detailed and rigorous examination of the existing literature reveals many inconsistencies that, unless resolved, could seriously hinder further progress in our field. The purpose of this review is to constructively expose, analyze and discuss a set of anomalies present in the current RT methodology, including: (a) the often inappropriate and misleading terminology used, (b) the need to clarify the aims of RT, (c) the very concept of maximal strength, (d) the control and monitoring of the resistance exercise dose, (e) the existing programming models and (f) the evaluation of training effects. A thorough and unbiased examination of these deficiencies could well lead to the adoption of a revised paradigm for RT. This new paradigm must guarantee a precise knowledge of the loads being applied, the effort they involve and their effects. To the best of our knowledge, currently this can only be achieved by monitoring repetition velocity during training. The main contribution of a velocity-based RT approach is that it provides the necessary information to know the actual training loads that induce a specific effect in each athlete. The correct adoption of this revised paradigm will provide coaches and strength and conditioning professionals with accurate and objective information concerning the applied load (relative load, level of effort and training effect). This knowledge is essential to make rational and informed decisions and to improve the training methodology itself.

A novel equation that incorporates the linear and hyperbolic nature of the force-velocity relationship in lower and upper limb exercises

The purpose of this study is to provide a force-velocity (F-V) equation that combines a linear and a hyperbolic region, and to compare its derived results to those obtained from linear equations. A total of 10 cross-training athletes and 14 recreationally resistance-trained young men were assessed in the unilateral leg press (LP) and bilateral bench press (BP) exercises, respectively. F-V data were recorded using a force plate and a linear encoder. Estimated maximum isometric force (F₀), maximum muscle power (P_max), and maximum unloaded velocity (V₀) were calculated using a hybrid (linear and hyperbolic) equation and three different linear equations: one derived from the hybrid equation (linear_hyb), one applied to data from 0 to 100% of F₀ (linear_0-100), and one applied to data from 45 to 100% of F₀ (linear_45-100). The hybrid equation presented the best fit to the recorded data (R² = 0.996 and 0.998). Compared to the results derived from the hybrid equation in the LP, significant differences were observed in F₀ derived from linear_0-100; V₀ derived from linear_hyb, linear_0-100 and linear_45-100; and P_max derived from linear_hyb and linear_45-100 (all p < 0.05). For the BP, compared to the hybrid equation, significant differences were found in F₀ derived from linear_0-100; and V₀ and P_max derived from linear_hyb, linear_0-100 and linear_45-100 (all p < 0.05). An F-V equation combining a linear and a hyperbolic region showed to fit adequately recorded F-V data from ~ 20 to 100% of F₀, and overcame the limitations shown by linear equations while providing relevant results.

Effectiveness of individualized training based on force-velocity profiling on physical function in older men

The study aimed to investigate the effectiveness of an individualized power training program based on force–velocity (FV) profiling on physical function, muscle morphology, and neuromuscular adaptations in older men. Forty‐nine healthy men (68 ± 5 years) completed a 10‐week training period to enhance muscular power. They were randomized to either a generic power training group (GPT) or an individualized power training group (IPT). Unlike generic training, individualized training was based on low‐ or high‐resistance exercises, from an initial force–velocity profile. Lower‐limb FV profile was measured in a pneumatic leg‐press, and physical function was assessed as timed up‐and‐go time (TUG), sit‐to‐stand power, grip strength, and stair‐climbing time (loaded [20kg] and unloaded). Vastus lateralis morphology was measured with ultrasonography. Rate of force development (RFD) and rate of myoelectric activity (RMA) were measured during an isometric knee extension. The GPT group improved loaded stair‐climbing time (6.3 ± 3.8 vs. 2.3% ± 7.3%, p = 0.04) more than IPT. Both groups improved stair‐climbing time, sit to stand, and leg press power, grip strength, muscle thickness, pennation angle, fascicle length, and RMA from baseline (p < 0.05). Only GPT increased loaded stair‐climbing time and RFD (p < 0.05). An individualized power training program based on FV profiling did not improve physical function to a greater degree than generic power training. A generic power training approach combining both heavy and low loads might be advantageous through eliciting both force‐ and velocity‐related neuromuscular adaptions with a concomitant increase in muscular power and physical function in older men.

Validity of a simple sit-to-stand method for assessing force-velocity profile in older adults

BACKGROUND

Lower limb muscle strength is an important determinant of physical function in older adults. However, its measure in clinical settings is limited because of the requirement for large-scale and costly equipment. A new simple protocol based on sit-to-stand test (STS) is developed to measure force velocity (F-v) and power velocity (P-v) profile in the community-dwelling older adults.

OBJECTIVE

The objective of this study was to assess the validity of this new methodology for measuring F-v and P-v profile compared to the gold standard isokinetic BIODEX.

PARTICIPANTS

46 older people aged 65-85 years (M = 73.7; SD = 7.7).

METHODS

F-v and P-v profiles were assessed in participants on their dominant leg. The concurrent validity of STS was tested using Spearman's rank correlation coefficient and Passing Bablok: maximal power output Pmax, optimal velocity and force Vopt and Fopt, maximal force at null velocity F0, maximal unloaded velocity V0 and coefficient of F-v (S_FV) and P-v equation (a_poly, b_poly).

RESULTS

No proportional difference for F0 and b_poly and a low significant correlation for Pmax (r = 0.314), S_fv (r = 0.229), a_poly (r = 0.335) and b_poly (r = 0.226) whereas the other parameters were non correlated significantly.

CONCLUSION

STS method is moderately reliable on force and power parameters whereas further improvements are needing for velocity parameters. However, its feasibility, portability and lower cost compared to other methods makes it very affordable in clinical context and will allow easy investigation of aging population.

Neuromuscular adaptations after 12 weeks of light- vs. heavy-load power-oriented resistance training in older adults

This study aimed to determine the specific adaptations provoked by power-oriented resistance training using light (LL-PT, 40% 1-RM) vs. heavy (HL-PT, 80% 1-RM) loads in older adults. Using a randomized within-subject study design, 45 older adults (>65 years) completed an 8-week control period (CTR) followed by 12 weeks of unilateral LL-PT vs. HL-PT on a leg press. The 1-RM, theoretical force at zero velocity (F₀ ), maximal unloaded velocity (V₀ ), and maximal muscle power (P_max ) were determined through a force-velocity relationship test. Isometrically, the rate of force development (RFD) and the corresponding muscle excitation of the knee extensor muscles were assessed. In addition, muscle cross-sectional area (CSA) and architecture of two quadriceps muscles were determined. Changes after CTR, LL-PT and HL-PT were compared using linear mixed models. HL-PT provoked greater improvements in 1-RM and F₀ (effect size (ES) = 0.55-0.68; p < 0.001) than those observed after LL-PT (ES = 0.27-0.47; p ≤ 0.001) (post hoc treatment effect, p ≤ 0.057). By contrast, ES of changes in V₀ was greater in LL-PT compared to HL-PT (ES = 0.71, p < 0.001 vs. ES = 0.39, p < 0.001), but this difference was not statistically significant. Both power training interventions elicited a moderate increase in P_max (ES = 0.65-0.69, p < 0.001). Only LL-PT improved early RFD (ie, ≤100 ms) and muscle excitation (ES = 0.36-0.60, p < 0.05). Increased CSA were noted after both power training programs (ES = 0.13-0.35, p < 0.035), whereas pennation angle increased only after HL-PT (ES = 0.37, p = 0.004). In conclusion, HL-PT seems to be more effective in improving the capability to generate large forces, whereas LL-PT appears to trigger greater gains in movement velocity in older adults. However, both interventions promoted similar increases in muscle power as well as muscle hypertrophy.

Reduced knee extensor torque production at low to moderate velocities in postmenopausal women with knee osteoarthritis

This study aimed to determine deficits in knee extensor muscle function through the torque-time and torque-velocity relationships and whether these deficits are associated with reduced functional performance in postmenopausal women with knee osteoarthritis (KOA). A clinical sample of postmenopausal women with established KOA (n = 18, ≥55 years) was compared to an age-matched healthy control sample (CON) (n = 26). The deficits in different parameters of the knee extensor torque-time (maximal isometric torque and rate of torque development) and torque-velocity relationship (maximum muscle power, maximal velocity and torque at 0-500°·s^-1 ) were assessed through a protocol consisting of isometric, isotonic and isokinetic tests. Functional performance was evaluated with sit-to-stand and stair-climbing tasks using a sensor-based technology (ie, time- and power-based outcomes). Postmenopausal women with KOA showed reduced maximal isometric torque (Hedge's g effect size (g) = 1.05, p = 0.001) and rate of torque development (g = 0.77-1.17, all p ≤ 0.02), combined with impaired torque production at slow to moderate velocities (g = 0.92-1.70, p ≤ 0.004), but not at high or maximal velocities (g = 0.16, p > 0.05). KOA were slower (g = 0.81-0.92, p ≤ 0.011) and less powerful (g = 1.11-1.29, p ≤ 0.001) during functional tasks. Additionally, knee extensor deficits were moderately associated with power deficits in stair climbing (r = 0.492-0.659). To conclude, knee extensor muscle weakness was presented in postmenopausal women with KOA, not only as limited maximal and rapid torque development during isometric contractions, but also dynamically at low to moderate velocities. These deficits were related to impaired functional performance. The assessment of knee extensor muscle weakness through the torque-time and torque-velocity relationships might enable individual targets for tailored exercise interventions in KOA.

Force-velocity profiling in athletes: Reliability and agreement across methods

The aim of the study was to examine the test-retest reliability and agreement across methods for assessing individual force-velocity (FV) profiles of the lower limbs in athletes. Using a multicenter approach, 27 male athletes completed all measurements for the main analysis, with up to 82 male and female athletes on some measurements. The athletes were tested twice before and twice after a 2- to 6-month period of regular training and sport participation. The double testing sessions were separated by ~1 week. Individual FV-profiles were acquired from incremental loading protocols in squat jump (SJ), countermovement jump (CMJ) and leg press. A force plate, linear encoder and a flight time calculation method were used for measuring force and velocity during SJ and CMJ. A linear regression was fitted to the average force and velocity values for each individual test to extrapolate the FV-variables: theoretical maximal force (F0), velocity (V0), power (Pmax), and the slope of the FV-profile (SFV). Despite strong linearity (R2>0.95) for individual FV-profiles, the SFV was unreliable for all measurement methods assessed during vertical jumping (coefficient of variation (CV): 14-30%, interclass correlation coefficient (ICC): 0.36-0.79). Only the leg press exercise, of the four FV-variables, showed acceptable reliability (CV:3.7-8.3%, ICC:0.82-0.98). The agreement across methods for F0 and Pmax ranged from (Pearson r): 0.56-0.95, standard error of estimate (SEE%): 5.8-18.8, and for V0 and SFV r: -0.39-0.78, SEE%: 12.2-37.2. With a typical error of 1.5 cm (5-10% CV) in jump height, SFV and V0 cannot be accurately obtained, regardless of the measurement method, using a loading range corresponding to 40-70% of F0. Efforts should be made to either reduce the variation in jumping performance or to assess loads closer to the FV-intercepts. Coaches and researchers should be aware of the poor reliability of the FV-variables obtained from vertical jumping, and of the differences across measurement methods.